Drill bit

ABSTRACT

The present invention provides a drill bit ( 1 ), and a method for its manufacture, that is suitable for use within a casing drilling process. The drill bit ( 1 ) comprises a monolithic body ( 5, 6, 8 ) made from bronze, e.g. aluminium bronze or nickel aluminium bronze, and a relatively hard material mounted thereon. The hard material may comprise polycrystalline diamond (PDC), tungsten carbide or cubic boron nitride. In one embodiment the monolithic body comprises a crown ( 6 ) and a plurality of cutting members ( 5 ). The hard material may be mounted directly on the crown or, alternatively, on the cutting members ( 5 ). The described drill bit ( 1 ) facilitates the drilling of hard rock formations while still being capable of being drilled through by a standard oilfield drill bit.

The present invention relates to the field of hydrocarbon exploration. More specifically, the present invention concerns a drill bit suitable for use within a casing drilling process.

When drilling subterranean formations for the purpose of hydrocarbon exploration it is known in the art to initially drill a first section of a well bore having a first diameter and thereafter to remove the drill bit from the well bore. A first tubular member of lesser diameter, known as casing string, is then placed in the well bore and subsequently the annulus between the well bore and the outside of the casing string is filled with a cement. The purpose of the cement is to isolate certain of the subterranean strata from each other. The next stage of the operation involves passing a second drill bit, having a smaller diameter than the first, through the casing string so as to permit the drilling of a second section of the well bore beyond the previously attained depth of the first section. This sequence is repeated as many times as necessary, with smaller and smaller components, until the ultimate desired depth of the well bore is achieved.

Positioned at the end of each casing string is a rounded guiding component known as a shoe. Typically, the leading edge of the shoe is constructed from cement, to enable it to be easily drilled through by the next drill bit.

The cost of oil exploration particularly in offshore regions is extremely high. Thus it is in the interest of the operator to minimise the time taken to form a well bore. At great depths, the round trip time to pull out a drill bit and replace it with another one can be many hours. This “trip” time is seen as non-productive and wasteful, and a significant advantage can be gained, if, having drilled to target depth the drill bit does not have to be removed from the well bore. In this way, a trip could be saved.

A proposed solution known in the art is to attach the drill bit to the leading end of the casing string, drill to a target depth and then cement the casing string. Certain advances in recent years have rendered this solution more viable, including the provision of premium casing threads that are able to take the necessary drilling torque, and rotary top drives able to transmit the torque directly to the trailing end of a casing string are now commonplace.

A limiting factor in many of the traditional techniques known in the art is the form of the drill bit. By design drill bits are generally robust devices designed to withstand the rigours of a downhole environment. Traditionally they are made from so called hard materials such as steel or tungsten carbide matrix. If such a drill bit were to be employed within the above described casing string drilling technique then after cementing of the casing string the subsequent drill bit would have to pass through the first drill bit before exiting the end of the casing string. However, modern drill bits optimised for rock removal are generally unable to drill through the materials from which they themselves are constructed without sustaining a level of damage which would render the task of drilling the next section of rock formation impossible.

It is possible to drill through these traditional drill bits with a special tool known as a mill, but these tools are unable to effectively penetrate the subsequent rock formations and so the mill would have to be removed from the well bore and replaced with an appropriate drill bit. In these circumstances, the trip saving advantage gained by drilling with the casing string would be lost.

An alternative solution to the above problem is described within PCT application number PCT/GB99/01816. Here a casing drilling shoe is disclosed which is adapted for attachment to the casing string. The drilling shoe comprises an outer drilling section constructed from a relatively hard material e.g. steel, which incorporates a cutting structure made from polycrystalline diamond (PCD) or tungsten carbide matrix. The tool further comprises an inner section constructed from aluminium which is a material known to be readily drillable. The casing drilling shoe further comprises a means for controllably displacing the outer drilling section to a radial position whereby it does not interfere with any subsequent drilling of the shoe. This enables the aluminium inner section to be drilled through using a standard drill bit and subsequently penetrated by a reduced diameter casing string.

The casing drilling shoe described in PCT application number PCT/GB99/01816 is extremely expensive to produce due to the requirement to incorporate a displaceable outer drilling section. In practice, these tools are also found to be sensitive to vibration and so they are not particularly robust which can cause mechanical failure during the drilling process. A further drawback of the described tool is the use of aluminium for the inner section. As with steel, when aluminium is drilled it tends to form into long strands. These strands then wrap themselves around the secondary drill bit so reducing the efficiency of any subsequent drilling process. Furthermore, aluminium is easily eroded by the action of the drilling fluids which are necessarily used to clean the well bore.

An alternative casing string drill bit is described in PCT application number PCT/GB00/04936. The described casing string drill bit is again constructed from a combination of a relatively soft (drillable) material and a relatively hard material. In particular, the described drill bit comprises a crown upon which are mounted a plurality of cutting members. The crown and cutting members are constructed from a relatively soft material such as aluminium, copper, nickel or a brass alloy. The cutting members are substantially covered by fragments of a relatively hard material such as tungsten carbide, PCD or cubic boron nitride. When a second drill bit is employed to drill through the described casing string drill bit the second drill passes through the relatively soft material and causes the fragments of the relatively hard material to fall away from the crown of the tool. The fragments are then subsequently removed from the well bore by the circulating drilling fluid.

In practice, and as recognised within the application itself, direct application of the relatively hard coatings to the crown and cutting member materials is difficult and in some combinations are just not practical. For example, extremely hard tungsten carbide particles cannot easily be applied to crown sections when these are made from aluminium. It is also recognised within the application that the described drill bit is not readily suitable for drilling through hard formations. When deployed with hard formations the fragments of the relatively hard material tend to be sheared away from the crown section, thus quickly reducing the efficiency of the drilling process.

A third solution presented in the prior art is that described within PCT application number PCT/US2005/004106. The described casing bit system is of a similar form to that described within PCT application number PCT/GB00/04936 however the tool now comprises a steel alloy crown upon which are mounted a plurality of cutting structures made from PDC or tungsten carbide. To facilitate drilling through the casing bit its inner surface is profiled so as to be suitable for receiving a bespoke secondary drill bit having a complementary outer surface profile that matches the inner surface profile of the casing bit.

To avoid damage when drilling through the steel alloy crown the cutters of the secondary drill bit are required to have of a low blade height. However, as is recognised in the art, lower blade heights generally result in lower drilling rates. Thus the rate at which the subsequent subterranean formations can be drilled by the secondary drill bit of this system is reduced.

The document also teaches that it is advantageous for the drill through process to modify the cutting structures of the casing bit that are located within the region configured to be drilled through e.g. by making the average amount of abrasive material contained in these regions less than the average amount of abrasive material contained by each of the cutting elements within the peripheral region; or alternatively by forming the cutting elements in this region from a substantially carbide-free material. Such modifications to the casing bit however can be detrimental to the rate at which it can drill the subterranean formations and also the distance that the drill bit may drill before wearing out.

The requirement of a bespoke secondary drill bit to drill through the casing systems of PCT application number PCT/US2005/004106 has obvious disadvantages since in the absence of such a bit the drilling operation is required to be suspended. In such circumstances it is known for operators to simply employ a standard oilfield drill bit to which they have access to in order to attempt to drill through the casing bit. This often results in the mechanical failure of the standard drill bit leading to increased down time as the damaged drill bit and associated debris have to be removed from the well bore. Furthermore, the design of the bespoke secondary drill bit may not be ideally suited to the particular formations that need to be drilled in the section below the casing drill bit.

It is recognised in the present invention that considerable advantage is to be gained in the provision of a drill bit that is able to drill hard rock formations effectively, but which itself is capable of being drilled by standard oilfield drill bits.

It is therefore an object of an aspect of the present invention to obviate or at least mitigate the foregoing disadvantages of the drill bits known in the art.

SUMMARY OF INVENTION

According to a first aspect of the present invention there is provided a drill bit suitable for drilling a casing in a well bore, the drill bit comprising a monolithic body made from bronze and one or more areas of a relatively hard material mounted on the monolithic body so as to provide a cutter suitable for cutting earth or rock.

The term relatively hard material indicates that the material from which the cutter is made is harder than the material from which the monolithic body is made i.e. bronze.

By making the drill bit from bronze allows for the drill bit to be drilled through from the proximal side by a conventional drill bit suited to drill the next rock formation section. Furthermore, the employment of bronze has the further advantage that when drilled it reduces to shavings. These shavings are then carried away by a drilling fluid and so do not act as an obstruction to any subsequent apparatus that is advanced into the well.

It is preferable for the monolithic body to exhibit a yield strength of at least 45,000 psi although it is most preferable for the yield strength to be at least 60,000 psi. Such yield strengths ensure that the drill bit can be employed to drill through relatively hard formations.

Suitable materials for forming the monolithic body are aluminium bronze and nickel aluminium bronze.

The monolithic body preferably comprises a crown. The crown may comprise one or more flow by areas. Preferably the crown comprises a mechanical connecter suitable for attaching the drill bit to a drill string.

Optionally the monolithic body further comprises one or more cutting members. The cutting members may comprise a gauge pad. Preferably the gauge pads comprise one more recesses suitable for receiving gauge inserts.

The relatively hard material may be mounted on the crown although it is preferable for the relatively hard material to be mounted on the leading edge of the one or more cutting members.

It is preferable for the relatively hard material to comprise one or more preformed cutters mounted within recesses. When the drill bit is drilled through the preformed cutters form fragments that are then carried away by a drilling fluid and so do not act as an obstruction to any subsequent apparatus that is advanced into the well.

The relatively hard material may comprise PDC. Alternatively, the relatively hard material may comprise tungsten carbide or cubic boron nitride.

According to a second aspect of the present invention there is provided a drill bit suitable for drilling a casing in a well bore, the drill bit comprising a monolithic body made from a copper alloy and one or more areas of a relatively harder material mounted on the monolithic body so as to provide a cutter suitable for cutting earth or rock.

Embodiments of the second aspect of the invention may comprise features to implement the preferred or optional features of the first aspect of the invention or vice versa.

According to a third aspect of the present invention there is provided a method of manufacturing a drill bit suitable for drilling a casing in a well bore, the method comprising:

-   -   casting a bronze so as to form a monolithic body;     -   heat treating the cast bronze;     -   mounting a relatively hard material on the monolithic body so as         to provide a means for cutting earth or rock.

The casting of the monolithic body may comprise casting a crown. The crown may comprise one or more flow by areas. Optionally casting of the monolithic body may comprise casting one or more cutting members. The cutting members may comprise a gauge pad. Preferably the gauge pads comprise one more recesses suitable for receiving gauge inserts.

The casting of the bronze may comprise casting an aluminium bronze or nickel aluminium bronze.

The casting of the bronze may comprise a casting method selected from the group comprising sand casting, centrifugal casting, continuous casting, permanent mould casting and plaster casting.

The heat treating of the cast bronze may provide the monolithic body with a yield strength of at least 45,000 psi, although it is preferable for the yield strength to be at least 60,000 psi.

The mounting of the relatively hard material on the monolithic body may comprise the formation of one or more recesses. The recesses may be formed on a leading edge of the one or more cutting members. Alternatively, the recesses may be formed on the crown.

The mounting of the relatively hard material on the monolithic body may further comprise the step of attaching preformed cutters within the one or more recesses. Preferably attaching the preformed cutters comprises brazing the cutters within the one or more recesses.

Preferably the method of manufacturing a drill bit further comprises forming a mechanical connection on the heat treated monolithic body.

According to a fourth aspect of the present invention there is provided a method of manufacturing a drill bit suitable for drilling a casing in a well bore, the method comprising:

-   -   casting a copper alloy so as to form a monolithic body;     -   heat treating the cast copper alloy;     -   mounting a relatively hard material on the monolithic body so as         to provide a means for cutting earth or rock.

Embodiments of the fourth aspect of the invention may comprise features to implement the preferred or optional features of the third aspect of the invention or vice versa.

BRIEF DESCRIPTION OF DRAWINGS

Aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the following drawings in which:

FIG. 1 presents a top view of a drill bit in accordance with an embodiment of the present invention;

FIG. 2 presents a side view of the drill bit of FIG. 1; and

FIG. 3 presents a cross section through the drill bit, as presented in FIG. 2.

DETAILED DESCRIPTION

A drill bit 1 in accordance with an aspect of the present invention will now be described with reference to FIGS. 1 to 3. The drill bit 1 is shown mounted to one end of a cylindrical body 2 via a first thread end connection 3 suitable for mating with the body 2. The opposite end of the cylindrical body 2 is then mounted on the lower end of a casing string (not shown) via a second thread end connection 4 suitable for mating with the casing.

The drill bit 1 itself comprises five cutting members 5 (often referred to as blades) located at the opposite end of the drill bit 1 to the first thread end connection 3, namely at the end where a crown 6 is located. It will be appreciated that the number of cutting members 5 may be varied, as deemed appropriate for the drilling operation. The cutting members 5 extend out from the crown 6 and each cutting member 5 can be seen to comprise a plurality of preformed cutters 7 located within corresponding recesses along a “leading edge” of the cutting members 5. In this respect the “leading edge” refers to the side of the cutting members 5 which directly contacts the ground or rock formation upon rotation of the drill bit 1. The preformed cutters 7 are produced from a relatively hard material (relative to the material from which the crown and the cutting members are made) and may be in the form of chips, or fragments of the hard material. The hard material may be tungsten carbide or cubic boron nitride although the preferred material is PCD. Further detail of how the preformed cutters 7 are attached to the cutting members 5 is provided below.

The cutting members 5 optionally comprise a gauge pads 8 arranged to extend away from the distal end of the drill bit 1, as are known in the art. In the presently described embodiment each gauge pad 8 can be seen to comprise a plurality of gauge inserts 9 located within corresponding recesses so as to provide physical protection to the gauge pad 8.

The crown 6 further comprises flow by areas 10 that allow for fluid circulated within the well bore to clean and lubricate the surfaces of the drill bit 1.

The body 2 may also comprises a stabiliser or centraliser (not shown), which maintains the drill bit 1 in the centre of the well bore during operation, and reaming members (not shown), which function to remove any irregularities or obstructions from the wall of the bore.

The crown 6, cutting members 5 and gauge pads 8 are formed as a monolithic or unitary body from a copper-based alloy so as to have a minimum yield strength of 45,000 psi although it is preferable for the yield strength to be at least 60,000 psi, as described in further detail below. The copper based alloy is preferably bronze and in particular aluminium bronze or nickel aluminium bronze, corresponding to material codes C95500 and C95800, respectively, within the Unified Numbering System (UNS).

It is recognised that whilst in the depicted embodiment the hard wearing cutting material is afforded to the leading edge of one or more cutting members 5 on the drill bit 1, the invention is not limited to this configuration. For example the preformed cutters 7 may be applied directly to the crown 6 in an embodiment having no cutting members 5. In a further alternative embodiment, a non-continuous layer of the hard cutting material may coat the crown 6 or the cutting members 5. In this instance, the surface of the crown 6 or the cutting members 5 will comprise areas that are not coated. However, upon rotation of the drill bit 1, the cumulative effect of the coated areas will be complete circumferential coverage of the diameter of the casing with which the drill bit 1 is located.

In a further alternative embodiment, the requirement for the cylindrical body 2 is removed and the drill bit 1 is mounted directly on the casing string, preferably via the first threaded mount 3.

In use, the drill bit 1 is run into a well bore (not shown) from the surface, typically whilst being rotated. The drill bit 1 guides a casing string (not shown) as it is advanced into the newly formed well bore to a predetermined depth. Upon reaching this depth, the casing is cemented to strengthen the lining of the bore.

If drilling beyond this first assembly is required, a second drill bit, and preferably a conventional drill bit suited to drill the next rock formation section, having a smaller diameter to the first is run from the surface into the well bore inside the casing string. Upon reaching the first assembly, the new drill bit can drill through the bronze material of the original drill bit 1, and therefore can proceed to a point beyond the depth reached by the original drill bit 1 within the well bore. It will be recognised that the drill bit 1 of an aspect of the present invention may also be employed as the second drill bit.

The nature of the bronze is such that when drilled it tends to form into small shavings, unlike steel and aluminium which tend to form into long strands. At the same time the hard material of the preformed cutters 7 is released from the crown 6 in the form of small fragments. The shavings and fragments are thus deposited into the well bore when the original bit 1 is drilled through and so do not obstruct the well bore or the secondary drill bit and are therefore not detrimental to the subsequent drilling process. In this manner a further section of the bore can be drilled beyond the previously attained depth without damage to the new drill bit and without needing to retrieve the first assembly from the bore.

Method of Manufacture

A method of manufacture of the drill bit 1 will now be described. In the first instance the bronze material is cast so as to form the monolithic or unitary body comprising the crown 6 and where present the cutting members 5 and the gauge pads 8. A sand casting method is preferably employed however alternative casting methods such as centrifugal, continuous, permanent mould or plaster casting techniques may be employed.

The monolithic body is then heat treated so as to provide the drill bit 1 with a yield strength of at least 45,000 psi, although it is preferable for the yield strength to be at least 60,000 psi. A hardening and tempering process known as TQ50 is the preferred heat treating process employed to achieve these desired yield strengths within the cast bronze material.

The next stage in the production of the drill bit 1 involves machining the hardened monolithic body so as to form the recesses for receiving the PDC cutters 7. The recesses may be formed directly on the crown 6 or upon the cutting members 5 as shown in the preferred embodiment of the invention. Where gauge pads 8 are also incorporated with the cutting members 5 appropriate recesses for the gauge inserts 9 are also machined at this stage.

Optionally the step of machining the hardened monolithic body comprises the production of a mechanical connection 3 on the hardened monolithic body so as to allow the drill bit 1 to be attached to a drill string. The mechanical connection may be a thread end connection.

Finally the cutters 7 are attached to the hardened monolithic body. In the preferred embodiment this involves the PDC cutters 7 being brazed within the machined recesses by means of a suitable braze alloy. Commercially available Argo-braze™ 49 is one such suitable braze alloy for fixing the PDC cutters 7 within the recesses of the hardened monolithic body. This is a specialised metal brazing filler comprising 49% Silver, 16% Copper, 23% Zinc, 4.5% Nickel and 7.5% Manganese. This brazing method provides a strong bond between the cutters 7 and the hardened monolithic body such that the cutters 7 do not get sheared from the hardened monolithic body during the operation of the drill bit 1. As a result the drill bit 1 is suitable for drilling through hard formations without any significant reduction in the efficiency of the drill bit.

The completed drill bit 1 can then be attached to the either the cylindrical body 2 or the casing string by means of the thread end connection 3 and employed for drilling a formation as previously described.

The present invention is inherent with significant advantages in that the time taken for the drilling operation can be greatly reduced as there is no need to implement complex and timely retrieval operations to recover apparatus from the well bore. As a result the profitable stage of production can be begin much sooner.

A further advantage is that unlike the drill bits known to the art, the drill bit of the present invention is drillable by another standard drill bit with the risk of damage to the standard drill bit being minimal. As a result there is no need to employ a bespoke drill bit to drill through the describe drill bit.

Further significant advantages of the present invention reside in the use of a bronze alloy to produce the drill bit. In the first instance the bronze allow can be manufactured so as to exhibit the required yield strength and bonding strength with the cutters to make the drill bit suitable for drilling through hard formations. As described previously, when the bronze drill bit is drilled it reduces to shavings. At the same time the preformed cutters are reduced to fragments. The shavings and fragments are then carried away by the drilling fluid and so do not act as an obstruction to any subsequent apparatus that is advanced into the well.

The present invention provides a drill bit, and a method for its manufacture, that is suitable for use within a casing drilling process. The drill bit comprises a monolithic body made from bronze, e.g. aluminium bronze or nickel aluminium bronze, and a relatively hard material mounted thereon. The hard material may comprise polycrystalline diamond (PDC), tungsten carbide or cubic boron nitride. In one embodiment the monolithic body comprises a crown and a plurality of cutting members. The hard material may be mounted directly on the crown or, alternatively, on the cutting members. The described drill bit facilitates the drilling of hard rock formations while still being capable of being drilled through by a standard oilfield drill bit.

The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims. 

1. A drill bit suitable for drilling a casing in a well bore, the drill bit comprising a monolithic body made from bronze and one or more areas of a relatively hard material mounted on the monolithic body so as to provide a cutter suitable for cutting earth or rock.
 2. The drill bit as claimed in claim 1 wherein the monolithic body exhibits a yield strength of at least 45,000 psi.
 3. The drill bit as claimed in claim 1 wherein the monolithic body exhibits yield strength of at least 60,000 psi.
 4. The drill bit as claimed in claim 1 wherein the monolithic body is formed from an aluminium bronze.
 5. The drill bit as claimed in claim 1 wherein the monolithic body is formed from a nickel-aluminium bronze.
 6. The drill bit as claimed in claim 1 wherein the monolithic body comprises a crown.
 7. The drill bit as claimed in claim 6 wherein the crown comprises one or more flow by areas.
 8. The drill bit as claimed in claim 6 wherein the crown comprises a mechanical connecter suitable for attaching the drill bit to a drill string.
 9. The drill bit as claimed in claim 1 wherein the monolithic body comprises one or more cutting members.
 10. The drill bit as claimed in claim 9 wherein the one or more cutting members comprise a gauge pad.
 11. The drill bit as claimed in claim 10 wherein every gauge pad comprises one or more recesses suitable for receiving gauge inserts.
 12. The drill bit as claimed in claim 6 wherein the relatively hard material is mounted on the crown.
 13. The drill bit as claimed in claim 9 wherein the relatively hard material is mounted on the leading edge of the one or more cutting members.
 14. The drill bit as claimed in claim 1 wherein the relatively hard material comprise one or more preformed cutters mounted within recesses.
 15. The drill bit as claimed in claim 1 wherein the relatively hard material comprises polycrystalline diamond.
 16. The drill bit as claimed in claim 1 wherein the relatively hard material comprises tungsten carbide.
 17. The drill bit as claimed in claim 1 wherein the relatively hard material comprises cubic boron nitride.
 18. A method of manufacturing a drill bit suitable for drilling a casing in a well bore, the method comprising: casting a bronze so as to form a monolithic body; heat treating the cast bronze; mounting a relatively hard material on the monolithic body so as to provide a means for cutting earth or rock.
 19. The method of manufacturing a drill bit as claimed in claim 18 wherein the casting of the monolithic body comprises casting a crown.
 20. The method of manufacturing a drill bit as claimed in claim 19 wherein the casting of the crown comprises the formation of one or more flow by areas.
 21. The method of manufacturing a drill bit as claimed in claim 18 wherein the casting of the monolithic body comprising casting one or more cutting members.
 22. The method of manufacturing a drill bit as claimed in claim 21 wherein the casting of the one or more cutting members comprises casting of a gauge pad.
 23. The method of manufacturing a drill bit as claimed in claim 22 wherein the casting of the gauge pad comprises casting one or more recesses suitable for receiving gauge inserts.
 24. The method of manufacturing a drill bit as claimed in claim 18 wherein the casting of a bronze comprises casting an aluminium bronze or a nickel aluminium bronze.
 25. The method of manufacturing a drill bit as claimed in claim 18 wherein the casting of the bronze comprises a casting method selected from the group comprising sand casting, centrifugal casting, continuous casting, permanent mould casting and plaster casting.
 26. The method of manufacturing a drill bit as claimed in claim 18 wherein the heat treating of the cast bronze provides the monolithic body with a yield strength of at least 45,000 psi.
 27. The method of manufacturing a drill bit as claimed in claim 18 wherein the heat treating of the cast bronze provides the monolithic body with a yield strength of at least 60,000 psi.
 28. The method of manufacturing a drill bit as claimed in claim 18 wherein the mounting of the relatively hard material on the monolithic body comprises the formation of one or more recesses.
 29. The method of manufacturing a drill bit as claimed in claim 28 wherein the one or more recesses are formed on a leading edge of one or more cutting members.
 30. The method of manufacturing a drill bit as claimed in claim 28 wherein the one or more recesses are formed on a crown.
 31. The method of manufacturing a drill bit as claimed in claim 28 wherein the mounting the relatively hard material on the monolithic body comprises attaching preformed cutters within the one or more recesses.
 32. The method of manufacturing a drill bit as claimed in claim 31 wherein attaching the preformed cutters comprises brazing the cutters within the one or more recesses.
 33. The method of manufacturing a drill bit as claimed in claim 18 wherein the method further comprises forming a mechanical connection on the heat treated monolithic body. 